Linking galaxy structural properties and star formation activity to black hole activity with IllustrisTNG

Habouzit, Mélanie; Genel, Shy; Somerville, Rachel S.; Kocevski, Dale; Hirschmann, Michaela; Dekel, Avishai; Choi, Ena; Nelson, Dylan; Pillepich, Annalisa; Torrey, Paul; Hernquist, Lars; Vogelsberger, Mark; Weinberger, Rainer; Springel, Volker

We study the connection between active galactic nuclei (AGN) and their host galaxies through cosmic time in the large-scale cosmological IllustrisTNG simulations. We first compare BH properties, i.e. the hard X-ray BH luminosity function, AGN galaxy occupation fraction, and distribution of Eddington ratios, to available observational constraints. The simulations produce a population of BHs in good agreement with observations, but we note an excess of faint AGN in hard X-ray (L_x̃ 10^{43-44} erg/s), and a lower number of bright AGN (L_x\gt 10^{44} erg/s), a conclusion that varies quantitatively but not qualitatively with BH luminosity estimation method. The lower Eddington ratios of the 109 M_{\odot } BHs compared to observations suggest that AGN feedback may be too efficient in this regime. We study galaxy star formation activity and structural properties, and design sample-dependent criteria to identify different galaxy types (star-forming/quiescent, extended/compact) that we apply both to the simulations and observations from the CANDELS fields. We analyse how the simulated and observed galaxies populate the specific star formation rate – stellar mass surface density diagram. A large fraction of the z = 0 M_{\star }≥slant 10^{11} M_{\odot } quiescent galaxies first experienced a compaction phase (i.e. reduction of galaxy size) while still forming stars, and then a quenching event. We measure the dependence of AGN fraction on galaxies’ locations in this diagram. After correcting the simulations with a redshift and AGN luminosity-dependent model for AGN obscuration, we find good qualitative and quantitative agreement with observations. The AGN fraction is the highest among compact star-forming galaxies (16-20{{ per cent}} at z ̃ 1.5-2), and the lowest among compact quiescent galaxies (6-10{{ per cent}} at z ̃ 1.5-2).

published in
Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 4, p.4413-4443, April 2019

links to paper